I'm just looking at the Viking Hird infantry on page 57 of the rule book and see they have spear and may upgrade to throwing spear. I've looked for the rules for spear and cannot find it. Page 24 discusses throwing spear and heavy throwing spear but not spear. I assume spear must be better than troops armed with just simple swords or knives?

I also read on Page 24 , right column bottom, that inferior fighters get a 5 to hit ONLY against superior fighters. is this correct?

In Swordpoint, a normal spear is the basic default infantry weapon, so there are no special rules or extra dice for troops armed with spears. Troops lacking spears are usually classed as Inferior Fighters.

Your second point is correct. Inferior Fighters hit Superior Fighters on a roll of 5, other troops as per normal.

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